1. Field of the Invention
The present invention is related to one method and apparatus for processing waste gas; more particularly, one method and apparatus for processing waste gas containing inorganic acid.
2. Description of Related Art
Due to a huge amount of inorganic acids (mainly hydrochloric acid, hydrofluoroic acid, sulfuric acid, and nitric acid) being used in the semi-conductor industry to process wafer washing with acid or etching, the manufacturers discharge waste gas and waste water that contain inorganic acids, which cause very serious pollution of the environment. Traditionally, the packed wet scrubber is commonly used to process the removing of inorganic acidic gas. During the process of gas cleaning, a huge amount of washing solution is sprayed on the packing so as to have the effect of gas-liquid contacting and removing inorganic acidic gas. Although the packed scrubber generally includes demister for removing droplets in the tail gas, it is difficult to avoid very small droplets being carried along the airflow. Utilizing a demister may retain the droplets of the tail gas, yet the retained droplets easily accumulate on top of the demister. During long-term operation, there is still small amount of inorganic acidic gas discharged along with the tail gas due to the washing solution itself or the retained droplets reaching the vapor-liquid equilibrium.
Presently, the existing devices of inorganic acidic wet scrubbers in the semi-conductor industry all have difficulty in achieving the requirement of 95% elimination rate. This environmental problem that the semiconductor industry faces is as the same as the problem needing to be solved in other high-tech industries such as the TFT-liquid crystal display industry.
These kinds of exhausts containing inorganic acid are generally treated in the wet scrubber by adding alkaline solution to absorb or wash the inorganic acid. Gas phase pollutants (inorganic acidic gas) dissolved in the liquid phase quickly form ions through acid-base neutralization. Thus, resistance of mass transfer from the gas phase to liquid phase is not produced. However, due to the concentration of inorganic acidic pollutants manufactured in semi-conductors and photoelectric devices being not high, the driving force for mass transfer (difference of concentrations) in the gas phase is not significant. Thus it is not able to produce greater diffusion mass transfer through the difference of concentrations between the gas phase and gas-liquid interface. Therefore, the traditional wet scrubber designed utilizing the two-film theory in chemical engineering has a poor efficiency in processing low concentration (<10 ppmv, specifically <5 ppmv) inorganic acidic exhausts.
There is no preferred method for processing the aforementioned low-concentration inorganic acidic gas, however, there have been a few techniques developed to avoid the drawbacks of the aforementioned wet scrubber. One is to utilize wet active carbon to adsorb sulfur dioxide (SO2) and hydrogen chloride (HCl) in waste gas. SO2 absorbed in the active carbon will be catalyzed and converted to sulfuric acid (H2SO4), and then washed by water. The exhausted H2SO4 solution will be concentrated and again infused into the active carbon bed. HCl absorbed in the active carbon will then be replaced by the concentrated H2SO4 solution. Wet scrubber and alkaline solution will then absorb and wash HCl. Another technique infuses hydroxide of alkali metal or carbonate into active carbon. Drying under high temperature (120 to 160° C.) is employed to make the potassium concentration of the active carbon increase to 5.9˜9.4%, and under such circumstances it has the ability to absorb SO2 and hydrogen cyanide (HCN). Also another technique proposes that active carbon with 20˜30 wt % calcium can remove gas containing chlorine or sulfur. However, the above-mentioned techniques have not mentioned the processing method for low concentration (<10 ppmv, especially <5 ppmv) inorganic acidic gas, nor the regeneration method of active carbon when its absorbency has been depleted.
In absorbent reuse relating techniques, there is one item of equipment for processing hydrogen sulfide gas in a wastewater treatment plant. The equipment utilizes an active carbon bed after being immersed in a sodium hydroxide solution as the adsorbent. The adsorbent bed has a thickness of 3 ft. When the adsorbability of the active carbon bed has been depleted, it requires an additional spare active carbon bed to alternatively function on-line, while the regeneration of adsorbability of active carbon bed is processed off-line. That is, the technique requires the active carbon bed to be offline for regeneration. Thus the stated equipment is not favorable for application in limited space. Moreover, the regeneration of the adsorbent bed requires five to fifteen days. In addition, the active carbon bed can only be regenerated for three to four times, resulting in short use life of the adsorbent.
It is therefore in great need to provide a long-term and stable method and apparatus for processing inorganic acidic gas to solve the problems of low concentration inorganic acidic gas in photoelectric device and semiconductor manufacturing.